Java Edition:Extra Health Silverfish
Silverfish with extra health are obtainable from their introduction in Beta 1.8 Pre-release until Beta 1.9 Prerelease. Normally, silverfish have 8 health (4 hearts) but during these versions silverfish spawned with 20 health (10 hearts). These silverfish can be updated up to 13w24a (1.6 snapshot), the discontinued feature is deleted after this version and the silverfish will go back to a normal health.
Obtaining
From Beta 1.8 Pre-release to Beta 1.9 Prerelease, silverfish spawn with 20 health (10 hearts). This was changed in Beta 1.9 Prerelease 2 so all silverfish spawned with 8 health. The eye of ender did not exist in these versions which can make locating a stronghold more difficult.
Observability
Extra health silverfish are destructively observable. Without an external tool, such as NBTExplorer, the only way to observe an extra health silverfish is through damaging it. If a silverfish is damaged ≥8 health and is still alive, the silverfish must have had extra health. However, when the extra health silverfish is discontinued it cannot heal this health back, so the discontinued feature is destroyed.